In user_account.liquid you would add:
{% highlight html %}
<p>
<label for="user_email">E-Mail</label>
<input type="text" name="user[email]" value="{{ current_user.email }}" />
</p>
<p>
<label for="user_phone">Phone</label>
<input type="text" name="user[phone]" value="{{ current_user.phone }}" />
</p>
{{ "{%" }} for customer_field in current_user.custom_fields %}
{{ "{%" }} include 'customer_field' with customer_field %}
{{ "{%" }} endfor %}
<p>
<input type="submit" name="submit" value="Save Changes" />
</p>
{% endhighlight %}
That will generate a form field for each custom field that the user has and put it after the standard form fields.
Note: there are no typos above. The partial and drop are called “customer_field” instead of “custom_field” to prevent ambiguity in filenames and drop names should we ever need to add custom fields to other areas of the application.
File: _customer_field.liquid
<p> <label for="user_{{ customer_field.field_name }}">{{ customer_field.label }}</label><br/> <input type="text" id="user_{{ customer_field.field_name }}" name="user[{{ customer_field.field_name }}]" value="{{ customer_field.value }}" /> </p>
<dl> <dt><label for="user_{{ customer_field.field_name }}">{{ customer_field.label }}</label></dt> <dd><input type="text" id="user_{{ customer_field.field_name }}" name="user[{{ customer_field.field_name }}]" value="{{ customer_field.value }}" /></dd> </dl>